It is known to control a handheld electronic apparatus or terminal, such as a mobile telephone or a handheld computer, by means of a conventional physical keyboard, individual physical alphanumerical keys and function keys, a thumb-wheel or a mouse. The use of keyboard and individual buttons integrated with the apparatus itself has the drawback that they are bulky per se and that space problems arise when the number of functions increases. Both a separate alphanumerical keyboard and a mouse have the drawbacks of constituting an extra component for the user and of requiring a separate work surface.
To alleviate these drawbacks electronic apparatus has been developed that can be at least partially controlled by means of a touchscreen and/or a touchpanel/touchpad. These input devices are available in analog or digital embodiments. Using a finger and/or a separate, handheld instrument—often termed “pen”—the user can select a point on the touch surface of the touchscreen or touchpanel for the purpose of selecting a corresponding point on the display area of the display unit. The user can thus activate various functions, such as “virtual” buttons for entering text, dialing numbers, activating functions, etc. Movement of a finger across the touch surface can result in a corresponding movement of a cursor across the display area of the display unit. However, no cursor need exist and instead the virtual function keys may be illuminated, for instance, when corresponding areas on the display area are activated.
A touchscreen is characterised in that it is transparent and entirely or partially covers the display area of the display unit. The user presses or clicks directly on the touch surface with his finger or with a pen, at the virtual buttons or fields indicated by the display unit, and no movable cursor is therefore necessary. A touchpanel or touchpad is characterised in that it does not coincide with the display area and is often used to control some form of cursor across the display area.
It is also known to make touchscreens and touchpads sensitive to the degree of pressure so that, by pressing sufficiently hard on the touch surface, a user can activate, i.e. “click on”, a selected button or function.
Known touchscreens and touchpads can to a certain extent alleviate the above-mentioned drawbacks of alphanumerical keyboards, physical keys, mouse and thumb-wheel, but they entail several other problems:                1. A touchscreen impairs the contrast and resolution of the picture since it is applied in front of the display area. This problem is particularly manifest in reflective display units where the light passes twice through the touchscreen. It is particularly difficult to provide glass or plastic with the conducting layer necessary for function of the touchscreen, without the transparency of the touchscreen being impaired.        2. Another drawback with touchscreens is that a part of the display area is obscured by the user's own finger.        3. A further drawback with touchscreens and touchpads is that the positioning accuracy is limited as a result of the large dimension of the finger in relation to the touch surface, a drawback that is particularly manifest with small touch surfaces. This problem can be partially alleviated by using a separate, thin pen to replace the finger but this has the disadvantage of requiring an extra component for the user to keep track of. The drawback of poor contrast is not alleviated, however, and even a pen obscures the display area to a certain extent.        4. Another important drawback of touchscreens is that they become dirty or scratched by the finger being passed over the surface, which further impairs visibility. Placing a separate scratch protection layer on top of the touchscreen is no solution since it would further impair the contrast.        5. A general drawback with pressure-sensitive touchscreens is that the touch surface must be flexible in its depth direction and the display area as a whole therefore becomes sensitive to external influence.        6. A touchscreen or touchpad is normally constructed from two layers held at a relative distance from each other by edge spacers. A problem is that the outer layer, constituting the touch surface, is tensioned and is therefore stretched when pressed. This problem is particularly prominent when the distance between the edge spacers is small, and especially close to these. This also entails the drawback that the pressure-sensitivity varies across the touch surface.        
U.S. Pat. No. 5,729,219 (Motorola Inc.) describes a handheld pager having a casing, a display unit arranged on the front of the casing and a flat touch surface parallel to the display area of the display unit on the opposite, rear side of the casing. The distance between the screen and the parallel touch surface is equal to the thickness of the casing. The screen and touch surface are equal in size and movement of the user's finger across the panel from left to right results in a corresponding left-to-right movement of a cursor on the screen, and vice versa. A finger movement on the touch surface results in a cursor movement of equal distance on the screen.